This invention is in the field of printed wiring boards.
The primary structure for assembling circuitry in the past was the use of a printed wiring board having through-holes for mounting components with attached leads. This technique imposes constraints on the component density that can be achieved. This restriction is in direct conflict with the trend toward smaller system packages that necessitate higher component density.
To relieve the leaded-package limitations, the industry has developed surface mount technology for IC packaging. This in turn has caused several problems for the printed wiring board. The first of these problems is increased thermal energy that must be dissipated. Since the density of components has increased, then there is more heat generated in a given surface area than there was using leaded technology. Therefore the printed wiring board should provide some means for transporting the additional heat flux.
It is an object of the invention to provide a printed wiring board that functions as a heat-sink.
It is also an object of the invention to provide a structure allowing good thermal conductivity from an internal thermal plane to the exterior of the printed wiring board for removal of heat.
Another heat related problem is that of the difference in thermal expansion coefficients between the components and the printed wiring board. The temperature change will cause the individual components and the printed wiring board to expand at different rates. As there is a high probability that there is difference in the rate of expansion, then the solder joint between the component and the printed wiring board will be stressed. Also, deflection of the printed wiring board will produce stress in the solder joints.
It is therefore an object of the invention to have a printed wiring board structure that possesses similar thermal expansion characteristics to surface mount components that are attached.
It is also an object of the invention to have a bonding pad on a printed wiring board that compensates for thermal expansion mis-matches and deflections between the printed wiring board and components that are attached to the printed wiring board.
Another problem that exists with higher component density is the increase in interconnects. This leads to difficulty in manufacturing the printed wiring board.
It is an object of the invention to allow a high degree of interconnectivity between components.
A related problem to interconnects is that many printed wiring boards (particularly with surface mount components) are designed with components on both sides of the board. However there are problems with attempting to connect the two sides of the printed wiring board. Drilling through the entire board takes up valuable real-estate as well as vastly complicates a multilayer printed wiring board design. Wrap-a-round connections are complex, are expensive, limit the number of side-to-side connections possible, and are easily damaged in handling.
It is an object of the invention to provide a simple, economical form of side-to-side interconnects.
Manufacture of prior-art multi-layer printed wiring boards with internal thermal planes has involved building two separate printed wiring boards, and then bonding them to the thermal plane. This creates a number of problems. First there is an alignment problem in insuring that the two printed wiring boards align with respect to each other. Second is the problem of side to side interconnects. Either a wrap-a-round, or a drill-through technique is normally employed. Another problem is weight, in that all three elements (the two printed wiring boards and the thermal plane) have to be self-supporting.
It is an object of the invention to provide a method of manufacture of a printed wiring board in which a thermal plane is used as a tooling plate for the printed wiring board.
It is also an object of the invention to provide a manufacturing process for a printed wiring board that allows simultaneous build of both sides of a double sided printed wiring board.
It is a further object of the invention to provide a technique for building a printed wiring board that minimizes alignment problems.
It is still another object of the invention to provide a low-weight, rigid printed wiring board having an internal thermal plane.
These and other objects of the invention are achieved by: a printed wiring board having a top and bottom surface, said surfaces having a plurality of mounting pads, said printed wiring board further including:
a central, substantially rigid core with a top and bottom face;
said core having at least one conductive through-hole, said through-hole being electrically insulated with respect to said core;
said through-hole being electrically coupled to a first conductive region overlying said top face and to a second electrically conductive region overlying said bottom face;
said core being further overlaid with alternating insulating and patterned conducting layers;
selected portions of said patterned conductive layers being electrically coupled to said first and second conductive regions by conductive vias extending through said insulating layers; and
selected ones of said mounting pads on said top and bottom surfaces being electrically coupled to said selected portions of said patterned conductive layers whereby an electrical interconnect extends from said top surface to said bottom surface.